U.S. patent application number 14/267428 was filed with the patent office on 2014-11-06 for nanoscale precursors for synthesis of fe2(si,ge)(s,se)4 crystalline particles and layers.
This patent application is currently assigned to Delaware State University. The applicant listed for this patent is Cheng-Yu Lai, Daniela Rodica Radu. Invention is credited to Cheng-Yu Lai, Daniela Rodica Radu.
Application Number | 20140326316 14/267428 |
Document ID | / |
Family ID | 51840783 |
Filed Date | 2014-11-06 |
United States Patent
Application |
20140326316 |
Kind Code |
A1 |
Radu; Daniela Rodica ; et
al. |
November 6, 2014 |
Nanoscale Precursors for Synthesis Of Fe2(Si,Ge)(S,Se)4 Crystalline
Particles and Layers
Abstract
Thin films comprising crystalline Fe.sub.2XY.sub.4, wherein X is
Si or Ge and Y is S or Se, are obtained by coating an ink comprised
of nanoparticle precursors of Fe.sub.2XY.sub.4 and/or a
non-particulate amorphous substance comprised of Fe, X and Y on a
substrate surface and annealing the coating. The coated substrate
thereby obtained has utility as a solar absorber material in thin
film photovoltaic devices.
Inventors: |
Radu; Daniela Rodica;
(Hockessin, DE) ; Lai; Cheng-Yu; (Hockessin,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Radu; Daniela Rodica
Lai; Cheng-Yu |
Hockessin
Hockessin |
DE
DE |
US
US |
|
|
Assignee: |
Delaware State University
Dover
DE
|
Family ID: |
51840783 |
Appl. No.: |
14/267428 |
Filed: |
May 1, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61817890 |
May 1, 2013 |
|
|
|
Current U.S.
Class: |
136/261 ;
252/519.4; 423/324; 423/511; 438/93 |
Current CPC
Class: |
H01L 21/02422 20130101;
H01L 21/02491 20130101; H01L 21/02628 20130101; Y02E 10/50
20130101; H01L 31/072 20130101; H01L 21/02568 20130101; H01L
21/0257 20130101; H01L 21/02601 20130101; H01L 31/032 20130101;
C09D 11/52 20130101; H01L 21/02425 20130101 |
Class at
Publication: |
136/261 ;
252/519.4; 423/324; 423/511; 438/93 |
International
Class: |
H01L 31/032 20060101
H01L031/032; H01L 31/18 20060101 H01L031/18 |
Claims
1. An ink comprising: a) a plurality of nanoparticles which are
precursors to a crystalline thin film comprised of
Fe.sub.2XY.sub.4, wherein X is selected from the group consisting
of Si, Ge and combinations thereof and Y is selected from the group
consisting of S, Se and combinations thereof; and b) a vehicle.
2. The ink of claim 1, wherein the nanoparticles comprise
nanoparticles of FeS.sub.2 and nanoparticles of elemental Ge and/or
elemental Si.
3. The ink of claim 1, wherein the nanoparticles comprise
nanoparticles of FeS.sub.2 and nanoparticles of GeS.
4. The ink of claim 1, wherein the nanoparticles comprise
nanoparticles of Fe.sub.2XS.sub.4, wherein X is selected from the
group consisting of Si, Ge and combinations thereof.
5. The ink of claim 1, wherein all of the nanoparticles have a
particle size of less than 200 nm.
6. A coated substrate comprising: a) a substrate; and b) a layer
disposed on the substrate comprising an ink in accordance with
claim 1.
7. A method of providing a crystalline thin film comprised of
Fe.sub.2XY.sub.4, wherein X is selected from the group consisting
of Si, Ge and combinations thereof and Y is selected from the group
consisting of S, Se and combinations thereof, on a substrate,
comprising the steps of: a) coating the substrate with at least one
layer of an ink in accordance with claim 1; and b) heating the
layer for a time and at a temperature effective to convert the
plurality of nanoparticles into the crystalline thin film comprised
of Fe.sub.2XY.sub.4.
8. A method of making an ink, comprising the steps of: a) combining
a source of Fe, a source of X, a source of Y and a capping agent to
form a reaction mixture; b) heating the reaction mixture for a time
and at a temperature effective to form nanoparticles of
Fe.sub.2XY.sub.4 or an admixture of nanoparticles of FeY2 and
nanoparticles of elemental X, wherein X is selected from the group
consisting of Si, Ge and combinations thereof and Y is selected
from the group consisting of S, Se and combinations thereof; and c)
combining the nanoparticles with a vehicle to form the ink.
9. The method of claim 8, comprising the additional steps after
step b) and before step c) of combining the reaction mixture with a
non-solvent to precipitate the nanoparticles, isolating the
precipitated nanoparticles and redispersing the isolated
nanoparticles.
10. An ink comprising: a) a non-particulate amorphous substance
comprised of Fe, X and Y which is a precursor to a crystalline thin
film of Fe.sub.2XY.sub.4, wherein X is selected from the group
consisting of Si, Ge and combinations thereof and Y is selected
from the group consisting of S, Se and combinations thereof; and b)
a vehicle.
11. A coated substrate comprising: a) a substrate; and b) a layer
disposed on the substrate comprising an ink in accordance with
claim 10.
12. A method of providing a crystalline thin film comprised of
Fe.sub.2XY.sub.4, wherein X is selected from the group consisting
of Si, Ge and combinations thereof and Y is selected from the group
consisting of S, Se and combinations thereof, on a substrate,
comprising the steps of: a) coating the substrate with at least one
layer of an ink in accordance with claim 10; and b) heating the
layer for a time and at a temperature effective to convert the
non-particulate amorphous substance into the crystalline thin film
comprised of Fe.sub.2XY.sub.4.
13. A method of making an ink, comprising the steps of: a)
combining a source of Fe, a source of X, a source of Y and a
capping agent to form a reaction mixture; b) heating the reaction
mixture for a time and at a temperature effective to form a
non-particulate amorphous substance comprised of Fe, X and Y,
wherein X is selected from the group consisting of Si, Ge and
combinations thereof and Y is selected from the group consisting of
S, Se and combinations thereof; and c) combining the
non-particulate amorphous substance with a vehicle to form the
ink.
14. The method of claim 13, comprising the additional steps after
step b) and before step c) of combining the reaction mixture with a
non-solvent to precipitate the non-particulate amorphous substance,
isolating the precipitated non-particulate amorphous substance and
redispersing the isolated non-particulate amorphous substance.
15. A method of making nanoparticles useful as precursors for
forming crystalline thin films comprised of Fe.sub.2XY.sub.4,
comprising the steps of: a) combining a source of Fe, a source of
X, a source of Y and a capping agent to form a reaction mixture; b)
heating the reaction mixture for a time and at a temperature
effective to form nanoparticles comprised of Fe.sub.2XY.sub.4 or an
admixture of nanoparticles of X and nanoparticles of FeY.sub.2,
wherein X is selected from the group consisting of Si, Ge and
combinations thereof and Y is selected from the group consisting of
S, Se and combinations thereof.
16. A method for making a nanoprecursor mixture useful for
preparing a thin film comprised of crystalline Fe.sub.2SiS.sub.4,
wherein the method comprises the following steps: a) forming a
reaction mixture comprised of an Fe compound, elemental S, and
elemental Si in a long chain alkyl amine; b) heating the reaction
mixture for about 0.2 to about 5 hours, optionally under an inert
atmosphere, at a temperature of from about 175.degree. C. to about
300.degree. C. to form a nanoparticulate reaction product comprised
of nanoparticles of elemental Si and nanoparticles of FeS.sub.2; c)
precipitating the nanoparticles formed in step b) by combining a
first anti-solvent with the nanoparticulate reaction product; d)
isolating the precipitated nanoparticles from step c); e)
redispersing the isolated nanoparticles from step d) in a solvent;
f) reprecipitating the redispersed nanoparticles from step e) by
combining a second anti-solvent, which may be the same as or
different from the first anti-solvent, with the redispersed
nanoparticles from step e); and g) isolating the precipitated
nanoparticles from step f).
17. A method for making a nanoprecursor mixture useful for
preparing a thin film comprised of crystalline Fe.sub.2GeS.sub.4,
wherein the method comprises the following steps: a) forming a
reaction mixture comprised of an Fe compound, elemental S, and a Ge
compound in a long chain alkyl amine; b) heating the reaction
mixture for about 0.2 to about 5 hours, optionally under an inert
atmosphere, at a temperature of from about 175.degree. C. to about
300.degree. C. to form a nanoparticulate reaction product comprised
of nanoparticles of elemental Ge and nanoparticles of FeS.sub.2; c)
precipitating the nanoparticles formed in step b) by combining a
first anti-solvent with the nanoparticulate reaction product; d)
isolating the precipitated nanoparticles from step c); e)
redispersing the isolated nanoparticles from step d) in a solvent;
f) reprecipitating the redispersed nanoparticles from step e) by
combining a second anti-solvent, which may be the same as or
different from the first anti-solvent, with the redispersed
nanoparticles from step e); and g) isolating the precipitated
nanoparticles from step f).
18. A nanoparticle comprising Fe, X and Y, wherein X is Si or Ge
and Y is S or Se.
19. The nanoparticle of claim 18, additionally comprising at least
one capping agent.
20. The nanoparticle of claim 18, wherein the molar ratio of Fe:X:Y
is about 2:1:4.
21. An admixture comprised of nanoparticles of FeY.sub.2 and
nanoparticles of elemental X, wherein Y is selected from the group
consisting of S, Se and combinations thereof and X is selected from
the group consisting of Si, Ge and combinations thereof.
22. The admixture of claim 21, additionally comprising at least one
capping agent.
23. The admixture of claim 21, wherein the molar ratio of Fe:X:Y is
about 2:1:4.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional
Application No. 61/817,890, filed May 1, 2013, the disclosure of
which is incorporated herein by reference in its entirety for all
purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to nanoparticles and
non-particulate amorphous substances that are useful precursors for
the preparation of crystalline films on substrates that contain
materials such as Fe.sub.2SiS.sub.4 and Fe.sub.2GeS.sub.4 in thin
film crystalline form. Such coated substrates may be utilized in
the manufacture of photovoltaic devices and the like.
BACKGROUND OF THE INVENTION
[0003] Three forms of thin-film solar panels have been developed
and commercialized in the last decade by identifying materials that
are both efficient absorbers of solar power and cost-effective for
manufacturer and consumer. These materials are: amorphous silicon
(a-Si), cadmium telluride (CdTe) and CIGS (copper indium gallium
sulfo-selenide). Although they operate effectively in thin-film
(1-3 microns) form, there are both environmental and economic
concerns for the cost and sustainability of the materials and
processes employed in these approaches.
[0004] An alternative solution was seen in pursuing sustainable
photovoltaic (PV) materials composed of Earth-abundant elements
such as Cu.sub.2ZnSn(S,Se).sub.4 (copper zinc tin sulfide-CZTS or
sulfo-selenide CZTSSe) or FeS.sub.2 (iron sulfide) for the absorber
layer. CZTSSe, benefiting from CIGS similarities, has already
proved itself at efficiencies >12%. However, photovoltaic
research using FeS.sub.2 absorbers still reports very low
efficiencies (.about.2%) despite this material's potential
comparable to a-Si, CdS and CIGS (>20%).
[0005] The use of Fe in PV was proposed more than 25 years ago in
the form of FeS.sub.2. FeS.sub.2 (also called pyrite or "fool's
gold") is an indirect band gap semiconductor with sustainable
composition of abundant elements. Unfortunately, the performance
problems associated with this material as a PV absorber are still
not fully understood.
[0006] The appeal of FeS.sub.2, in addition to the material's low
cost and abundance, has been that it exhibits a useful band gap
(E.sub.g=0.9 eV) and an absorption coefficient above 10.sup.5 at
E.sub.g+0.1 eV. This high absorption coefficient makes FeS.sub.2
unique among inorganic materials, allowing downsizing the thickness
of the absorber layer to lower than 0.1 .mu.m in a solar cell able
to capture most of the incident solar radiation. The attractiveness
of this thickness is visible when compared to 1.5-3.0 .mu.m for
current thin-film technologies and >200 .mu.m for single-crystal
Si cells. Such thin layers not only conserve material, but they
also provide an avenue to high efficiency through efficient charge
separation associated with a high internal electrical field.
However, the promise of FeS.sub.2 as a "golden" solution for PV has
not come true to date.
[0007] Recently, a large team of scientists from NREL and Oregon
State University has investigated the phenomena related to lack of
performance in FeS.sub.2 and pointed to an intrinsic thermal
instability of the material along with considerable challenges that
must be surmounted for production of high-quality, single-phase
FeS.sub.2 films. This work is reported in Yu et al., Advanced
Energy Materials 1(5), 748-753 (2011). To circumvent the problem,
they have used the following design principle: "select systems that
do not spontaneously phase-separate into sulfur (S) deficient
conducting materials with small band-gaps." In order to provide a
ligand-field splitting of sufficient magnitude for effective solar
absorption the Fe.sup.2+ ion must be bound by at least six S atoms
thus assuring a sufficiently large band gap. This generally
requires Fe.sup.2+ in an octahedral site. Adding a third element
with an electronegativity that favors strong covalent bonding with
sulfur can stabilize such a site. From these considerations, this
research group has chosen Fe.sub.2SiS.sub.4 and Fe.sub.2GeS.sub.4
for investigation. The analytical evaluations (summarized in Table
1, which is adapted from the aforementioned article in Advanced
Energy Materials) led to the conclusion that the two materials are
suitable to successfully deliver the performance originally
expected from FeS.sub.2.
TABLE-US-00001 TABLE 1 Fe.sub.2SiS.sub.4 and Fe.sub.2GeS.sub.4
evaluation results TGA Enthalpy of Mass Calculated Measured Decom-
Loss Calculated Direct Direct position Starting Absorption Bandgap
Bandgap in Binary Point Coefficient Material (eV) (eV) Sulfides
(eV) (.degree. C.) (cm.sup.-1) Fe.sub.2SiS.sub.4 1.55 1.54 0.59
1000 >10.sup.5 Fe.sub.2GeS.sub.4 1.4 1.36 0.64 725
>10.sup.5
[0008] The thermal stability of the two materials along with their
close to ideal bandgap for solar cell fabrication makes the two
materials good candidates for achieving the initial promise of
FeS.sub.2.
[0009] To date, however, methods for preparing devices containing
crystalline Fe.sub.2SiS.sub.4 and Fe.sub.2GeS.sub.4 and the like as
an absorber layer using nano-scale precursors have not been
available.
SUMMARY OF THE INVENTION
[0010] One aspect of the invention provides an ink comprising:
[0011] a) a plurality of nanoparticles which are precursors to a
crystalline thin film comprised of Fe.sub.2XY.sub.4, wherein X is
selected from the group consisting of Si, Ge and combinations
thereof and Y is selected from the group consisting of S, Se and
combinations thereof; and [0012] b) a vehicle.
[0013] For example, the nanoparticles may comprise nanoparticles of
FeS.sub.2 and nanoparticles of elemental Ge or elemental S. In
another embodiment of the invention, the nanoparticles may comprise
nanoparticles of FeS.sub.2 and nanoparticles of GeS. In yet another
embodiment, the nanoparticles may comprise nanoparticles of
Fe.sub.2XS.sub.4, wherein X is selected from the group consisting
of Si, Ge and combinations thereof. The vehicle may be a solvent,
in particular an organic solvent. According to one embodiment, all
of the nanoparticles may have a particle size of less than 200
nm.
[0014] Further provided by the present invention is a coated
substrate comprising: [0015] a) a substrate; and [0016] b) a layer
disposed on the substrate comprising an ink in accordance with the
above description.
[0017] Still another aspect of the invention provides a method of
providing a crystalline thin film comprised of Fe.sub.2XY.sub.4,
wherein X is selected from the group consisting of Si, Ge and
combinations thereof and Y is selected from the group consisting of
S, Se and combinations thereof, on a substrate, comprising the
steps of: [0018] a) coating the substrate with at least one layer
of an ink in accordance with the above description; and [0019] b)
heating the layer for a time and at a temperature effective to
convert the plurality of nanoparticles into the crystalline thin
film comprised of Fe.sub.2XY.sub.4.
[0020] Additionally provided by the present invention is a method
of making nanoparticles useful as precursors for forming
crystalline thin films comprised of Fe.sub.2XY.sub.4, comprising
the steps of: [0021] a) combining a source of Fe, a source of X, a
source of Y and a capping agent (e.g., a long chain alkyl amine) to
form a reaction mixture; [0022] b) heating the reaction mixture for
a time and at a temperature effective to form nanoparticles
comprised of Fe.sub.2XY.sub.4 or an admixture of nanoparticles of X
(which may be in elemental form) and nanoparticles of FeY.sub.2,
wherein X is selected from the group consisting of Si, Ge and
combinations thereof and Y is selected from the group consisting of
S, Se and combinations thereof.
[0023] This method may optionally comprise an additional step after
step b) of recovering the nanoparticles from the reaction mixture.
To prepare an ink suitable for coating onto a substrate, the
recovered nanoparticles may optionally be combined with a vehicle,
such as an organic solvent.
[0024] The aforementioned method for making nanoparticles may
comprise the additional steps after step b) of combining the
reaction mixture with a non-solvent (anti-solvent) to precipitate
the nanoparticles, isolating the precipitated nanoparticles and
redispersing the isolated nanoparticles (for example, in a
solvent).
[0025] Another aspect of the present invention pertains to an ink
comprising: [0026] a) a non-particulate amorphous substance
comprised of Fe, X and Y which is a precursor to a crystalline thin
film of Fe.sub.2XY.sub.4, wherein X is selected from the group
consisting of Si, Ge and combinations thereof and Y is selected
from the group consisting of S, Se and combinations thereof; and
[0027] b) a vehicle.
[0028] A coated substrate may also be provided by the present
invention comprising: [0029] a) a substrate; and [0030] b) a layer
disposed on the substrate comprising an ink containing a
non-particulate amorphous substance in accordance with the above
description.
[0031] A still further aspect of the invention furnishes a method
of providing a crystalline thin film comprised of Fe.sub.2XY.sub.4,
wherein X is selected from the group consisting of Si, Ge and
combinations thereof and Y is selected from the group consisting of
S, Se and combinations thereof, on a substrate, comprising the
steps of: [0032] a) coating the substrate with at least one layer
of an ink containing a non-particulate amorphous substance in
accordance with the foregoing description; and [0033] b) heating
the layer for a time and at a temperature effective to convert the
non-particulate amorphous substance into the crystalline thin film
comprised of Fe.sub.2XY.sub.4.
[0034] A method of making an ink is provided in another aspect of
the invention, comprising the steps of: [0035] a) combining a
source of Fe, a source of X, a source of Y and a capping agent
(e.g., a long chain alkyl amine) to form a reaction mixture; [0036]
b) heating the reaction mixture for a time and at a temperature
effective to form a non-particulate amorphous substance comprised
of Fe, X and Y, wherein X is selected from the group consisting of
Si, Ge and combinations thereof and Y is selected from the group
consisting of S, Se and combinations thereof; and [0037] c)
combining the non-particulate amorphous substance with a vehicle to
form the ink.
[0038] This method may comprise the additional steps after step b)
and before step c) of combining the reaction mixture with a
non-solvent to precipitate the non-particulate amorphous substance,
isolating the precipitated non-particulate amorphous substance and
redispersing the isolated non-particulate amorphous substance.
[0039] Also provided by the present invention is a nanoparticle
comprising, consisting essentially of, or consisting of Fe, X and
Y, wherein X is selected from the group consisting of Si, Ge and
combinations thereof and Y is selected from the group consisting of
S, Se and combinations thereof. For example, the nanoparticle may
comprise, consist essentially of, or consist of a) Fe, Si and S, b)
Fe, Si and Se, c) Fe, Ge and S, or d) Fe, Ge and Se. The
nanoparticle may be additionally comprised of a capping agent. The
molar ratio of Fe:X:Y in the nanoparticle may be about 2:1:4. For
example, the nanoparticle may be a nanoparticle of
Fe.sub.2SiS.sub.4, Fe.sub.2GeS.sub.4, Fe.sub.2SiSe.sub.4, or
Fe.sub.2GeSe.sub.4.
[0040] Additionally provided by the present invention is an
admixture comprised of nanoparticles of FeY.sub.2 and nanoparticles
of elemental X, wherein Y is selected from the group consisting of
S, Se and combinations thereof and X is selected from the group
consisting of Si, Ge and combinations thereof. The admixture may
additionally comprise at least one capping agent. The molar ratio
of Fe:X:Y in the admixture may about 2:1:4. For example, the
admixture may comprise nanoparticles of FeS.sub.2 and nanoparticles
of elemental Si or Ge.
DESCRIPTION OF THE FIGURE
[0041] FIG. 1 shows, in schematic form, a perspective view of a
stack of layers in a solar cell.
DETAILED DESCRIPTION OF THE INVENTION
Definitions of Certain Terms
[0042] Herein, the terms "solar cell" and "photovoltaic cell" are
synonymous unless specifically defined otherwise. These terms refer
to devices that use semiconductors to convert visible and
near-visible light energy into usable electrical energy. The terms
"band gap energy," "optical band gap," and "band gap" are
synonymous unless specifically defined otherwise. These terms refer
to the energy required to generate electron-hole pairs in a
semiconductor material, which in general is the minimum energy
needed to excite an electron from the valence band to the
conduction band.
[0043] Herein, the term "nanoparticles" is meant to include
nanoparticles with a variety of shapes that are characterized by an
average longest dimension of about 1 nm to about 500 nm. Herein, by
nanoparticle "size" or "size range" or "size distribution," we mean
that the average longest dimension of a plurality of nanoparticles
falls within the range. "Longest dimension" is defined herein as
the measurement of a nanoparticle from end to end. The "longest
dimension" of a particle will depend on the shape of the particle.
For example, for particles that are roughly or substantially
spherical, the longest dimension will be a diameter of the
particle. For other particles, the longest dimension will be a
diagonal or a side.
[0044] Herein, the term "crystalline" is meant to refer to a
material or substance which exhibits one or more peaks by x-ray
diffraction (XRD). As used herein, the term "crystalline" includes
both fully and partially crystalline materials or substances.
[0045] Herein, the term "amorphous" means a material or substance
that does not exhibit at least one peak by x-ray diffraction
(XRD).
Inks
[0046] One aspect of this invention an ink comprising, consisting
essentially of, or consisting of: [0047] a) nanoparticles and/or a
non-particulate amorphous substance which are precursors to a
crystalline thin film comprised of Fe.sub.2XY.sub.4, wherein X is
selected from the group consisting of Si, Ge and combinations
thereof and Y is selected from the group consisting of S, Se and
combinations thereof; and [0048] b) a vehicle.
[0049] The nanoparticles or non-particulate amorphous substance of
component a) are selected to be capable of providing a crystalline
thin film comprised of Fe.sub.2XY.sub.4 when the ink is coated onto
a substrate surface and further processed in accordance with the
procedures described further herein. This ink may be considered an
Fe.sub.2XY.sub.4 precursor ink, as it contains one or more
precursors for forming a thin film containing crystalline
Fe.sub.2XY.sub.4 on the surface of a substrate.
Nanoparticles/Non-Particulate Amorphous Substances
[0050] The ink may contain a single type of nanoparticle or
non-particulate amorphous substance. That is, all the nanoparticles
present in the ink may have the same composition and/or the
entirety of the non-particulate amorphous substance may have the
same composition. For example, all of the nanoparticles may be
nanoparticles of Fe.sub.2GeS.sub.4, Fe.sub.2GeSe.sub.4,
Fe.sub.2SiS.sub.4 or Fe.sub.2SiSe.sub.4. Multiple types of
nanoparticles may also be present in the ink. In one embodiment,
mixtures of two or more types of nanoparticles selected from the
group consisting of Fe.sub.2GeS.sub.4, Fe.sub.2GeSe.sub.4,
Fe.sub.2SiS.sub.4 and Fe.sub.2SiSe.sub.4 are utilized. In another
embodiment, different types of nanoparticles are selected and
combined so as to separately provide the elemental components of
the desired crystalline thin film of Fe.sub.2XY.sub.4 to be formed
on a substrate. For example, nanoparticles of FeS.sub.2 may be
present in combination with nanoparticles of Ge (as precursors to
Fe.sub.2GeS.sub.4), nanoparticles of FeS.sub.2 may be present in
combination with nanoparticles of Si (as precursors to
Fe.sub.2SiS.sub.4), or nanoparticles of FeS.sub.2 may be present in
combination with nanoparticles of GeS (as precursors to
Fe.sub.2GeS.sub.4). Where different types of nanoparticles or
non-particulate amorphous substances are used in combination, their
relative amounts may be selected in accordance with the desired
stoichiometry of Fe, X and Y in the final crystalline thin film on
the coated substrate. As an example, in the embodiment where
nanoparticles of FeS.sub.2 and nanoparticles of Si are used in
combination, the molar ratio of FeS.sub.2:Si may be about 2:1 in
order to provide a crystalline thin film comprised of
Fe.sub.2SiS.sub.4.
[0051] The nanoparticles can have an average longest dimension of
less than about 500 nm, 400 nm, 300 nm, 250 nm, 200 nm, 150 nm, or
100 nm, as determined by electron microscopy. In other embodiments,
all of the nanoparticles are less than 200 nm, less than 150 nm or
less than 100 nm in size with respect to their longest dimension.
The nanoparticles may take different forms, depending upon the
starting materials and treatment conditions; for example, the
nanoparticles may be spheroidal, block-shaped or in the form of
platelets. In at least certain embodiments of the invention, the
nanoparticles are at least partially crystalline. The nanoparticles
can be synthesized by techniques such as: decomposition and
reduction of metal salts and complexes; chemical vapor deposition;
electrochemical deposition; use of gamma-, x-ray, laser or
UV-irradiation; ultrasonic (sonication) or microwave treatment;
electron- or ion-beams; arc discharge; electric explosion of wires;
or biosynthesis.
[0052] The nanoparticles may, in certain embodiments of the
invention, comprise, consist essentially of, or consist of Fe, X
and Y, wherein X is selected from the group consisting of Si, Ge
and combinations thereof and Y is selected from the group
consisting of S, Se and combinations thereof. For example, the
nanoparticles may comprise, consist essentially of, or consist of
a) Fe, Si and S, b) Fe, Si and Se, c) Fe, Ge and S, or d) Fe, Ge
and Se. The nanoparticles may be additionally comprised of a
capping agent, as described below. The molar ratio of Fe:X:Y in the
nanoparticles may be about 2:1:4.
[0053] Illustrative embodiments of the invention include, for
example, nanoparticles of Fe.sub.2XY.sub.4 (e.g.,
Fe.sub.2SiS.sub.4, Fe.sub.2SiSe.sub.4, Fe.sub.2GeS.sub.4,
Fe.sub.2GeSe.sub.4), optionally in combination with one or more
capping agents; and admixtures of nanoparticles of X (Si, Ge) and
nanoparticles of FeY.sub.2 (Y=S, Se), optionally in combination
with one or more capping agents.
Capping Agent
[0054] In some embodiments, the nanoparticles and/or
non-particulate amorphous substance further comprise one or more
capping agents. The capping agent can aid in the dispersion of the
nanoparticles or non-particulate amorphous substance and can also
inhibit their interaction and agglomeration in the ink.
[0055] Suitable capping agents include, but are not limited to:
[0056] (a) Organic molecules that contain functional groups such as
N-, O-, S-, Se- or P-based functional groups;
[0057] (b) Lewis bases;
[0058] (c) Amines, thiols, selenols, phosphine oxides, phosphines,
phosphinic acids, pyrrolidones, pyridines, carboxylates,
phosphates, heteroaromatics, peptides, and alcohols;
[0059] (d) Alkyl amines (e.g., long chain alkyl amines, such as
C8-C22 saturated or unsaturated alkyl amines, e.g., oleylamine,
decylamine, dodecylamine, hexadecyclamine, octadecylamine), alkyl
thiols, alkyl selenols, trialkylphosphine oxide,
trialkylphosphines, alkylphosphonic acids, polyvinylpyrrolidone,
polycarboxylates, polyphosphates, polyamines, pyridine,
alkylpyridines, aminopyridines, peptides comprising cysteine and/or
histidine residues, ethanolamines, citrates, thioglycolic acid,
oleic acid, and polyethylene glycol;
[0060] (e) inorganic chalcogenides, including metal chalcogenides,
and zintl ions;
[0061] (f) S.sup.2-, Se.sup.2-, Se.sub.2.sup.2-, Se.sub.3.sup.2-,
Se.sub.4.sup.2-, Se.sub.6.sup.2-, Te.sub.2.sup.2-, Te.sub.3.sup.2-,
Te.sub.4.sup.2-, In.sub.2Se.sub.4.sup.2-, and
In.sub.2Te.sub.4.sup.2-, wherein the positively charged counterions
can be alkali metal ions, ammonium, hydrazinium, or
tetraalkylammonium;
[0062] (g) Degradable capping agents, including
dichalcogenocarbamates, monochalcogenocarbamates, xanthates,
trithiocarbonates, dichalcogenoimidodiphosphates, thiobiurets,
dithiobiurets, chalcogenosemicarbazides, and tetrazoles. These
capping agents can be degraded by thermal and/or chemical
processes, such as acid- and base-catalyzed processes. Degradable
capping agents include: dialkyl dithiocarbamates, dialkyl
monothiocarbamates, dialkyl diselenocarbamates, dialkyl
monoselenocarbamates, alkyl xanthates, alkyl trithiocarbonates,
disulfidoimidodiphosphates, diselenoimidodiphosphates, tetraalkyl
thiobiurets, tetraalkyl dithiobiurets, thiosemicarbazides,
selenosemicarbazides, tetrazole, alkyl tetrazoles,
amino-tetrazoles, thio-tetrazoles, and carboxylated tetrazoles. In
some embodiments, Lewis bases (e.g., amines) can be added to
nanoparticles stabilized by carbamate, xanthate, and
trithiocarbonate capping agents to catalyze their removal from the
nanoparticle;
[0063] (h) The solvent in which the nanoparticle or non-particulate
amorphous substance is formed, such as oleylamine; and
[0064] (i) Short-chain carboxylic acids, such as formic, acetic, or
oxalic acids.
[0065] The Lewis base can be chosen such that it has a boiling
temperature at ambient pressure that is greater than or equal to
about 200.degree. C., 150.degree. C., 120.degree. C., or
100.degree. C., and/or can be selected from the group consisting
of: organic amines, phosphine oxides, phosphines, thiols, and
mixtures thereof. In some embodiments, the capping agent comprises
a surfactant or a dispersant.
Volatile Capping Agents
[0066] In some embodiments, the nanoparticles and/or
non-particulate amorphous substance comprise a volatile capping
agent. A capping agent is considered volatile if, instead of
decomposing and introducing impurities when a composition or ink of
nanoparticles or non-particulate amorphous substance is formed into
a film, it evaporates during film deposition, drying or annealing.
Volatile capping agents include those having a boiling point less
than about 200.degree. C., 150.degree. C., 120.degree. C., or
100.degree. C. at ambient pressure. Volatile capping agents may be
adsorbed or bonded onto particles during synthesis or during an
exchange reaction. Thus, in one embodiment, nanoparticles, or an
ink or reaction mixture of nanoparticles stabilized by a first
capping agent, as incorporated during synthesis, are mixed with a
second capping agent that has greater volatility to exchange in the
particles the second capping agent for the first capping agent.
Suitable volatile capping agents include: ammonia, methyl amine,
ethyl amine, propylamine, butylamine, tetramethylethylene diamine,
acetonitrile, ethyl acetate, butanol, pyridine, ethanethiol,
propanethiol, butanethiol, t-butylthiol, pentanethiol, hexanethiol,
tetrahydrofuran, and diethyl ether. Suitable volatile capping
agents can also include: amines, amidos, amides, nitriles,
isonitriles, cyanates, isocyanates, thiocyanates, isothiocyanates,
azides, thiocarbonyls, thiols, thiolates, sulfides, sulfinates,
sulfonates, phosphates, phosphines, phosphites, hydroxyls,
hydroxides, alcohols, alcoholates, phenols, phenolates, ethers,
carbonyls, carboxylates, carboxylic acids, carboxylic acid
anhydrides, glycidyls, and mixtures thereof.
Preparation of Nanoparticles and Non-Particulate Amorphous
Substances
[0067] As previously mentioned, the ink may comprise nanoparticles
of Fe.sub.2XY.sub.4, such as nanoparticles of Fe.sub.2SiS.sub.4 or
Fe.sub.2GeS.sub.4; nanoparticles of FeY.sub.2 and nanoparticles of
X; and/or a non-particulate amorphous substance comprised of Fe, X
and Y. The nanoparticles and non-particulate amorphous substance
may be prepared in accordance with the following procedure.
Dispersions and/or solutions of a source of Fe, a source of Si
and/or Ge, and a source of S and/or Se are separately prepared by
combining each starting material with a capping agent such as
oleylamine to obtain an initial mixture and treating each initial
mixture with ultrasound while heating (e.g., at a temperature of
from about 70.degree. C. to about 150.degree. C.) to yield a
dispersion or solution (in some embodiments, colloids of the
starting materials are obtained). The dispersions/solutions are
then combined (in the stoichiometry needed) and the combined
dispersions/solutions (the reaction mixture) then heated for a time
(e.g., about 0.2 to about 10 hours) and at a temperature (e.g.,
about 150.degree. C. to about 300.degree. C.) effective to achieve
reaction of the starting materials, thereby forming the desired
nanoparticles and/or non-particulate amorphous substance. The
starting components may be stirred or otherwise agitated during
heating; microwaves may be used to assist during this heating step.
Generally speaking, longer heating times tend to favor the
formation of nanoparticles whereas shorter heating times tend to
favor the generation of the non-particulate amorphous substance.
Mixtures of nanoparticles and non-particulate amorphous substance
may result when intermediate reaction times are utilized. The
heating can be conducted under an inert atmosphere (e.g., an argon
atmosphere). The nanoparticles and/or non-particulate amorphous
substance can be isolated, for example, by precipitation by a
non-solvent followed by centrifugation, and can be further purified
by washing and repeated dispersion precipitation steps. In one
embodiment, the nanoparticles or non-particulate amorphous
substance are/is precipitated using a non-solvent which is a
mixture of aliphatic hydrocarbons and alcohol (e.g., a mixture of
hexanes and ethanol). In another embodiment, the
nanoparticles/non-particulate amorphous substance isolated by
precipitation are/is dispersed in a dispersion solvent such as an
aromatic hydrocarbon (e.g., toluene), prior to being
re-precipitated.
[0068] The sources of Fe, Si/Ge and S/Se may be in elemental or
compound form. A single starting material may function as a source
of more than one of the components needed to ultimately form the
desired nanoparticles or non-particulate amorphous substance.
Suitable sources of Fe include iron compounds such as, for example,
iron halides (e.g., FeCl.sub.2) and organoiron complexes (e.g.,
Fe(acac).sub.3). Suitable sources of Si and Ge include, for
example, elemental Si and Ge halides (such as GeI.sub.4). Suitable
sources of S and Se include, for example, elemental S. The sources
of Fe, Si/Ge and S/Se may be in nanoparticulate form or
non-nanoparticulate form. In one embodiment, a source of Fe, Si/Ge
or S/Se is initially in non-nanoparticulate form but then converted
to nanoparticulate form prior to being combined with the other
sources, using techniques such as sonication, heating milling,
grinding or the like. The relative amounts of the Fe source, the
Si/Ge source and the S/Se source may be selected to provide the
stoichiometry needed to attain the desired crystalline
Fe.sub.2XY.sub.4 in the final annealed coating. Accordingly, the
source of Fe, the source of Si/Ge and the source of S/Se may be
combined in amounts appropriate to provide a Fe:(Si/Ge):(S/Se)
stoichiometry of about 2:1:4.
[0069] One exemplary method for making a nanoprecursor mixture
useful for preparing a thin film comprised of crystalline
Fe.sub.2SiS.sub.4 comprises the following steps: [0070] a) forming
a reaction mixture comprised of an Fe compound (e.g., FeCl.sub.2),
elemental S, and elemental Si in a long chain alkyl amine (for
example, by combining a solution of FeCl.sub.2 in the long chain
alkyl amine, a solution of elemental S in the long chain alkyl
amine, and a dispersion of elemental Si (which may be in
nanoparticulate form) in the long chain alkyl amine); [0071] b)
heating the reaction mixture for about 0.2 to about 5 hours,
optionally under an inert atmosphere, at a temperature of from
about 175.degree. C. to about 300.degree. C. to form a
nanoparticulate reaction product comprised of nanoparticles of
elemental Si and nanoparticles of FeS.sub.2; [0072] c)
precipitating the nanoparticles formed in step b) by combining a
first anti-solvent (e.g., a hexanes:ethanol mixture) with the
nanoparticulate reaction product; [0073] d) isolating the
precipitated nanoparticles from step c) (by centrifugation, for
example); [0074] e) redispersing the isolated nanoparticles from
step d) in a solvent (e.g., toluene); [0075] f) reprecipitating the
redispersed nanoparticles from step e) by combining a second
anti-solvent (which may be the same as or different from the first
anti-solvent) with the redispersed nanoparticles from step e); and
[0076] g) isolating the precipitated nanoparticles from step f) (by
centrifugation, for example).
[0077] The amounts of Fe compound, S and Si utilized in step a) may
be selected to provide a molar ratio of Fe:Si:S of about 2:1:4.
Steps e)-g) may be repeated multiple times as may be needed or
desired in order to remove excess capping agent and/or other
substances from the nanoparticle mixture. The isolated nanoparticle
mixture may be combined with a vehicle, such as an organic solvent,
as well as one or more additional optional addives to provide an
ink suitable for coating onto a substrate and annealing to form a
thin film of crystalline Fe.sub.2SiS.sub.4 on the substrate
surface, as described elsewhere herein.
[0078] One exemplary method for making a nanoprecursor mixture
useful for preparing a thin film comprised of crystalline
Fe.sub.2GeS.sub.4 comprises the following steps: [0079] a) forming
a reaction mixture comprised of an Fe compound (e.g., FeCl.sub.2,
Fe(acac).sub.3), elemental S, and a Ge compound (e.g. GeI.sub.4) in
a long chain alkyl amine (for example, by combining a solution of
the Fe compound in the long chain alkyl amine, a solution of
elemental S in the long chain alkyl amine, and a solution of
elemental Ge in the long chain alkyl amine); [0080] b) heating the
reaction mixture for about 0.2 to about 5 hours, optionally under
an inert atmosphere, at a temperature of from about 175.degree. C.
to about 300.degree. C. to form a nanoparticulate reaction product
comprised of nanoparticles of elemental Ge and nanoparticles of
FeS.sub.2; [0081] c) precipitating the nanoparticles formed in step
b) by combining a first anti-solvent (e.g., a mixture of hexanes
and ethanol) with the nanoparticulate reaction product; [0082] d)
isolating the precipitated nanoparticles from step c) (by
centrifugation, for example); [0083] e) redispersing the isolated
nanoparticles from step d) in a solvent (e.g., toluene); [0084] f)
reprecipitating the redispersed nanoparticles from step e) by
combining a second anti-solvent (which may be the same as or
different from the first anti-solvent) with the redispersed
nanoparticles from step e); and [0085] g) isolating the
precipitated nanoparticles from step f) (by centrifugation, for
example).
[0086] The amounts of Fe compound, S and Ge compound utilized in
step a) may be selected to provide a molar ratio of Fe:Si:S of
about 2:1:4. Steps e)-g) may be repeated multiple times as may be
needed or desired in order to remove excess capping agent and/or
other substances from the nanoparticle mixture. The isolated
nanoparticle mixture may be combined with a vehicle, such as an
organic solvent, as well as one or more additional optional addives
to provide an ink suitable for coating onto a substrate and
annealing to form a thin film of crystalline Fe.sub.2GeS.sub.4 on
the substrate surface, as described elsewhere herein.
[0087] One exemplary method for making a nanoprecursor comprised of
Fe.sub.2GeS.sub.4 nanoparticles useful for preparing a thin film
comprised of crystalline Fe.sub.2GeS.sub.4 comprises the following
steps: [0088] a) forming a reaction mixture comprised of an Fe
compound (e.g., FeCl.sub.2, Fe(acac).sub.3), elemental S, and a Ge
compound (e.g. GeI.sub.4) in a long chain alkyl amine (for example,
by combining a solution of the Fe compound in the long chain alkyl
amine, a solution of elemental S in the long chain alkyl amine, and
a solution of elemental Ge in the long chain alkyl amine); and
[0089] b) heating the reaction mixture for about 0.2 to about 5
hours, optionally under an inert atmosphere and optionally while
exposing the reaction mixture to microwaves, at a temperature of
from about 175.degree. C. to about 300.degree. C. to form a
nanoparticulate reaction product comprised of nanoparticles of
Fe.sub.2GeS.sub.4.
[0090] The amounts of Fe compound, S and Ge compound utilized in
step a) may be selected to provide a molar ratio of Fe:Si:S of
about 2:1:4. The nanoparticles formed in step b) may be
precipitated by combining a first anti-solvent (e.g., a mixture of
hexanes and ethanol) with the nanoparticulate reaction product. The
precipitated nanoparticles may be isolated (by centrifugation, for
example). The isolated nanoparticles may be redispersed in a
solvent (e.g., toluene). The redispersed nanoparticles may be
reprecipitated by combining a second anti-solvent (which may be the
same as or different from the first anti-solvent) with the
redispersed nanoparticles. The precipitated nanoparticles may be
isolated again (by centrifugation, for example). These steps may be
repeated multiple times, if so desired. The nanoparticles of
Fe.sub.2GeS.sub.4 thereby obtained may be combined with a vehicle
to provide an ink.
[0091] A non-particulate amorphous substance useful as a precursor
for forming a thin film comprised of crystalline Fe.sub.2GeS.sub.4
on a substrate surface (to provide, for example, an absorber layer
in a photovoltaic device or the like) may be prepared in an
analogous manner according to the above-described procedure, except
that the reaction conditions of step b) are milder (e.g., a shorter
heating time, typically about 0.5 to about 1.5 hours).
Vehicle
[0092] The ink comprises a vehicle to carry the nanoparticles. The
vehicle is typically a fluid or a low-melting solid with a melting
point of less than about 100.degree. C., 90.degree. C., 80.degree.
C., 70.degree. C., 60.degree. C., 50.degree. C., 40.degree. C., or
30.degree. C. In some embodiments, the vehicle comprises one or
more solvents. Suitable solvents include, but are not limited to:
aromatics, heteroaromatics, alkanes, chlorinated alkanes, ketones,
esters, nitriles, amides, amines, thiols, selenols, pyrrolidinones,
ethers, thioethers, selenoethers, alcohols, water, and mixtures
thereof. Useful examples of these solvents include toluene,
p-xylene, mesitylene, benzene, chlorobenzene, dichlorobenzene,
trichlorobenzene, pyridine, 2-aminopyridine, 3-aminopyridine,
2,2,4-trimethylpentane, n-octane, n-hexane, n-heptane, n-pentane,
cyclohexane, chloroform, dichloromethane, 1,1,1-trichloroethane,
1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane, 2-butanone,
acetone, acetophenone, ethyl acetate, acetonitrile, benzonitrile,
N,N-dimethylformamide, butylamine, hexylamine, octylamine,
3-methoxypropylamine, 2-methylbutylamine, isoamylamine,
1-propanethiol, 1-butanethiol, 2-butanethiol,
2-methyl-1-propanethiol, t-butyl thiol, 1-pentanethiol,
3-methyl-1-butanethiol, cyclopentanethiol, 1-hexanethiol,
cyclohexanethiol, 1-heptanethiol, 1-octanethiol, 2-ethyhexanethiol,
1-nonanethiol, tert-nonyl mercaptan, 1-decanethiol,
mercaptoethanol, 4-cyano-1-butanethiol, butyl
3-mercaptoproprionate, methyl 3-mercaptoproprionate,
1-mercapto-2-propanol, 3-mercapto-1-propanol, 4-mercapto-1-butanol,
6-mercapto-1-hexanol, 2-phenylethanethiol, thiophenol,
N-methyl-2-pyrrolidinone, tetrahydrofuran, 2,5-dimethylfuran,
diethyl ether, ethylene glycol diethyl ether, diethylsulfide,
diethylselenide, 2-methoxyethanol, isopropanol, butanol, ethanol,
methanol and mixtures thereof.
[0093] In some embodiments, the wt % of the vehicle in the ink is
about 95 to about 5 wt %, 95 to 50 wt %, 95 to 60 wt %, 95 to 70 wt
%, 95 to 80 wt %, 90 to 10 wt %, 80 to 20 wt %, 70 to 30 wt %, 60
to 40 wt %, 98 to 50 wt %, 98 to 60 wt %, 98 to 70 wt %, 98 to 75
wt %, 98 to 80 wt %, 98 to 85 wt %, 95 to 75 wt %, 95 to 80 wt %,
or 95 to 85 wt % based upon the total weight of the ink. In some
embodiments, the vehicle may function as a dispersant or capping
agent, as well as being the carrier vehicle for the nanoparticles
and/or non-particulate amorphous substance. Solvent-based vehicles
that are particularly useful as capping agents comprise
heteroaromatics, amines, thiols, selenols, thioethers, or
selenoethers.
Additional Ink Components
[0094] In various embodiments, the ink may further comprise
additive(s), an elemental chalcogen, or mixtures thereof.
Additives
[0095] In some embodiments, the ink may further comprise one or
more additives. Suitable additives include dispersants,
surfactants, polymers, binders, ligands, capping agents, defoamers,
thickening agents, corrosion inhibitors, plasticizers, thixotropic
agents, viscosity modifiers, and dopants. In some embodiments,
additives are selected from the group consisting of: capping
agents, dopants, polymers, and surfactants. In some embodiments,
the ink comprises up to about 10 wt %, 7.5 wt %, 5 wt %, 2.5 wt %
or 1 wt % additives, based upon the total weight of the ink.
Suitable capping agents comprise the capping agents, including
volatile capping agents, described above.
Dopants
[0096] Suitable dopants include sodium and alkali-containing
compounds. In some embodiments, the alkali-containing compounds are
selected from the group consisting of: alkali compounds comprising
N-, O-, C-, S-, or Se-based organic ligands, alkali sulfides,
alkali selenides, and mixtures thereof. In other embodiments, the
dopant comprises an alkali-containing compound selected from the
group consisting of: alkali-compounds comprising amidos; alkoxides;
acetylacetonates; carboxylates; hydrocarbyls; O-, N-, S-, Se-,
halogen-, or tri(hydrocarbyl)silyl-substituted hydrocarbyls;
thiolates and selenolates; thio-, seleno-, and dithiocarboxylates;
dithio-, diseleno-, and thioselenocarbamates; and
dithioxanthogenates. Other suitable dopants include antimony
chalcogenides selected from the group consisting of antimony
sulfide and antimony selenide.
Polymers and Surfactants
[0097] Suitable polymeric additives include
vinylpyrrolidone-vinylacetate copolymers and (meth)acrylate
copolymers, including PVP/VA E-535 (International Specialty
Products), and Elvacite.RTM. 2028 binder and Elvacite.RTM. 2008
binder (Lucite International, Inc.). In some embodiments, polymers
can function as binders or dispersants.
[0098] Suitable surfactants comprise siloxy-, fluoryl-, alkyl-,
alkynyl-, and ammonium-substituted surfactants. These include, for
example, Byk.RTM. surfactants (Byk Chemie), Zonyl.RTM. surfactants
(DuPont), Triton.RTM. surfactants (Dow), Surfynol.RTM. surfactants
(Air Products), Dynol.RTM. surfactants (Air Products), and
Tego.RTM. surfactants (Evonik Industries AG). In certain
embodiments, surfactants function as coating aids, capping agents,
or dispersants.
[0099] In some embodiments, the ink comprises one or more binders
or surfactants selected from the group consisting of: decomposable
binders; decomposable surfactants; cleavable surfactants;
surfactants with a boiling point less than about 250.degree. C.;
and mixtures thereof. Suitable decomposable binders include: homo-
and co-polymers of polyethers; homo- and co-polymers of
polylactides; homo- and co-polymers of polycarbonates including,
for example, Novomer PPC (Novomer, Inc.); homo- and co-polymers of
poly[3-hydroxybutyric acid]; homo- and co-polymers of
polymethacrylates; and mixtures thereof. A suitable low-boiling
surfactant is Surfynol.RTM. 61 surfactant from Air Products.
Cleavable surfactants useful herein as capping agents include
Diels-Alder adducts, thiirane oxides, sulfones, acetals, ketals,
carbonates, and ortho esters. Suitable cleavable surfactants
include: alkyl-substituted Diels Alder adducts, Diels Alder adducts
of furans; thiirane oxide; alkyl thiirane oxides; aryl thiirane
oxides; piperylene sulfone, butadiene sulfone, isoprene sulfone,
2,5-dihydro-3-thiophene carboxylic acid-1,1-dioxide-alkyl esters,
alkyl acetals, alkyl ketals, alkyl 1,3-dioxolanes, alkyl
1,3-dioxanes, hydroxyl acetals, alkyl glucosides, ether acetals,
polyoxyethylene acetals, alkyl carbonates, ether carbonates,
polyoxyethylene carbonates, ortho esters of formates, alkyl ortho
esters, ether ortho esters, and polyoxyethylene ortho esters.
Elemental Chalcogen
[0100] In some embodiments, the ink comprises an elemental
chalcogen selected from the group consisting of sulfur, selenium,
and mixtures thereof. Useful forms of sulfur and selenium include
powders that can be obtained from Sigma-Aldrich (St. Louis, Mo.)
and Alfa Aesar (Ward Hill, Mass.). In some embodiments, the
chalcogen powder is soluble in the ink vehicle. If the chalcogen is
not soluble in the vehicle, its particle size can be 1 nm to 200
microns. In some embodiments, the particles have an average longest
dimension of less than about 100 microns, 50 microns, 25 microns,
10 microns, 5 microns, 4 microns, 3 microns, 2 microns, 1.5
microns, 1.25 microns, 1.0 micron, 0.75 micron, 0.5 micron, 0.25
micron, or 0.1 micron. Preferably, the chalcogen particles are
smaller than the thickness of the film that is to be formed. The
chalcogen particles can be formed by ball milling,
evaporation-condensation, melting and spraying ("atomization") to
form droplets, or emulsification to form colloids.
Ink Preparation
[0101] Preparing the ink typically comprises mixing the components
by any conventional method. In some embodiments, the preparation is
conducted under an inert atmosphere.
Substrate
[0102] The substrate onto which the ink is disposed can be rigid or
flexible. In one embodiment, the substrate comprises: (i) a base;
and (ii) optionally, an electrically conductive coating on the
base. The base material may selected from the group consisting of
glass, metals, ceramics, and polymeric films. Suitable base
materials include metal foils, plastics, polymers, metalized
plastics, glass, solar glass, low-iron glass, green glass,
soda-lime glass, metalized glass, steel, stainless steel, aluminum,
ceramics, metal plates, metalized ceramic plates, and metalized
polymer plates. In some embodiments, the base material comprises a
filled polymer (e.g., a polyimide and an inorganic filler). In some
embodiments, the base material comprises a metal (e.g., stainless
steel) coated with a thin insulating layer (e.g., alumina).
[0103] Suitable electrically conductive coatings include metal
conductors, transparent conducting oxides, and organic conductors.
Of particular interest are substrates of molybdenum-coated
soda-lime glass, molybdenum-coated polyimide films, and
molybdenum-coated polyimide films further comprising a thin layer
of a sodium compound (e.g., NaF, Na.sub.2S, or Na.sub.2Se).
Ink Deposition
[0104] The ink may be disposed on a substrate to provide a coated
substrate by any suitable method such as solution-based coating or
printing techniques, including spin-coating, spray-coating,
dip-coating, rod-coating, drop-cast coating, roller-coating,
slot-die coating, draw-down coating, ink-jet printing, contact
printing, gravure printing, flexographic printing, and screen
printing. The coating can be dried by evaporation, by applying
vacuum, by heating, or by combinations thereof. In some
embodiments, the substrate and disposed ink are heated at a
temperature effective to remove at least a portion of the solvent.
The drying step can be a separate, distinct step, or can occur as
the substrate and precursor ink are heated in an annealing step.
For example, an initial drying step could be carried out at a
relatively low temperature of from about 100.degree. C. to about
200.degree. C., followed by an annealing step at a substantially
higher temperature (e.g., about 500.degree. C. to about 600.degree.
C.).
Annealing
[0105] In some embodiments, the coated substrate is heated in an
annealing step at about 100-900.degree. C., 200-800.degree. C.,
250-800.degree. C., 300-800.degree. C., 400-700.degree. C., or
450-600.degree. C. The annealing conditions may be selected to
promote or achieve the formation of a layer comprised of
crystalline Fe.sub.2SiS.sub.4, Fe.sub.2GeS.sub.4,
Fe.sub.2SiSe.sub.4 or Fe.sub.2GeSe.sub.4 on the surface of the
substrate. In some embodiments, the coated substrate is heated for
a time in the range of about 1 min to about 48 h; 1 min to about 30
min; 10 min to about 10 h; 15 min to about 5 h; 20 min to about 3
h; or, 30 min to about 2 h. Typically, the annealing may comprise
thermal processing, rapid thermal processing (RTP), rapid thermal
annealing (RTA), pulsed thermal processing (PTP), laser beam
exposure, heating via IR lamps, electron beam exposure, pulsed
electron beam processing, heating via microwave irradiation, or
combinations thereof. Herein, RTP refers to a technology that can
be used in place of standard furnaces and involves single-wafer
processing, and fast heating and cooling rates. RTA is a subset of
RTP, and consists of unique heat treatments for different effects,
including activation of dopants, changing substrate interfaces,
densifying and changing states of films, repairing damage, and
moving dopants. Rapid thermal anneals are performed using either
lamp-based heating, a hot chuck, or a hot plate. PTP involves
thermally annealing structures at extremely high power densities
for periods of very short duration, resulting, for example, in
defect reduction. Similarly, pulsed electron beam processing uses a
pulsed high energy electron beam with short pulse duration. Pulsed
processing is useful for processing thin films on
temperature-sensitive substrates. The duration of the pulse is so
short that little energy is transferred to the substrate, leaving
it undamaged.
[0106] In some embodiments, the annealing is carried out under an
atmosphere comprising: an inert gas (nitrogen or a Group VIIIA gas,
particularly argon); optionally hydrogen; and optionally, a
chalcogen source such as selenium vapor, sulfur vapor, hydrogen
sulfide, hydrogen selenide, diethyl selenide, or mixtures thereof.
In some embodiments, the annealing is conducted with slow heating
and/or cooling steps, e.g., temperature ramps and declines of less
than about 15.degree. C. per min, 10.degree. C. per min, 5.degree.
C. per min, 2.degree. C. per min, or 1.degree. C. per min. In other
embodiments, the annealing is conducted with rapid and/or cooling
steps, e.g., temperature ramps and declines of greater than about
15.degree. C. per min, 20.degree. C. per min, 30.degree. C. per
min, 45.degree. C. per min, or 60.degree. C. per min.
Additional Layers
[0107] In some embodiments, the coated substrate further comprises
one or more additional layers. These one or more layers can be of
the same composition as the first layer comprised of crystalline
Fe.sub.2XY.sub.4 or can differ in composition. For example, a first
ink may be coated onto the surface of the substrate and dried to
provide a first layer, with a second ink then coated onto the top
surface of the first layer and then dried to provide a second
layer, with this procedure being repeated with additional inks to
form additional layers. The first, second and additional inks may
be the same as, or different from, each other. At least partial
annealing may be performed prior to placing successive layers onto
the substrate. Alternatively, annealing may be postponed until all
the desired layers of dried ink have been formed on the substrate
surface.
Coating and Film Thickness
[0108] By varying the ink concentration and/or coating technique
and temperature, layers of varying thickness can be coated in a
single coating step. In some embodiments, the coating thickness can
be increased by repeating the coating and drying steps. These
multiple coatings can be conducted with the same ink or with
different inks. As described above, wherein two or more inks are
mixed, the coating of multiple layers with different inks can be
used to fine-tune the stoichiometry and purity of the thin films
comprised of Fe.sub.2XY.sub.4. It can also be used to tune the
absorption of the film, e.g., by creating films with gradient
Fe.sub.2XY.sub.4 compositions. Soft-bake and annealing steps can be
carried out between the coating of multiple layers. In these
instances, the coating of multiple layers with different inks can
be used to create layers having different compositions and
characteristics. The coating of multiple layers can also be used to
fill in voids in the at least one layer and planarize or create an
underlayer to the at least one layer.
[0109] The annealed film typically has an increased density and/or
reduced thickness versus that of the wet precursor layer. In some
embodiments, the film thicknesses of the dried and annealed
coatings are 0.1-200 microns; 0.1-100 microns; 0.1-50 microns;
0.1-25 microns; 0.1-10 microns; 0.1-5 microns; 0.1-3 microns; 0.3-3
microns; or 0.5-2 microns.
Purification of Coated Layers and Films
[0110] Application of multiple coatings, washing the coating,
and/or exchanging capping agents can serve to reduce carbon-based
impurities in the coatings and films. For example, after an initial
coating, the coated substrate can be dried and then a second
coating can be applied and coated by spin-coating. The spin-coating
step can wash organics out of the first coating. Alternatively, the
coated film can be soaked in a solvent and then spun to wash out
the organics. Examples of useful solvents for removing organics in
the coatings include alcohols, e.g., methanol or ethanol, and
hydrocarbons, e.g., toluene. As another example, dip-coating the
substrate into the ink can be alternated with dip-coating the
coated substrate into a bath to remove impurities and capping
agents. Removal of non-volatile capping agents from the coating can
be further facilitated by exchanging these capping agents with
volatile capping agents. For example, the volatile capping agent
can be used as the washing solution or as a component in a bath. In
some embodiments, a layer of a coated substrate comprising a first
capping agent is contacted with a second capping agent, thereby
replacing the first capping agent with the second capping agent to
form a second coated substrate. Advantages of this method include
film densification along with lower levels of carbon-based
impurities in the film, particularly if and when it is later
annealed. Alternatively, binary sulfides and other impurities can
be removed by etching.
Preparation of Devices, Including Thin-Film Photovoltaic Cells
[0111] Various electrical elements that can be formed, at least in
part, by the use of the materials described herein include
electronic circuitry, resistors, capacitors, diodes, rectifiers,
electroluminescent lamps, memory elements, field effect
transistors, bipolar transistors, unijunction transistors, thin
film transistors, metal-insulator-semiconductor stacks, charge
coupled devices, insulator-metal-insulator stacks, organic
conductor-metal-organic conductor stacks, integrated circuits,
photodetectors, lasers, lenses, waveguides, gratings, holographic
elements, filters (e.g., add-drop filters, gain-flattening filters,
and cut-off filters), mirrors, splitters, couplers, combiners,
modulators, sensors (e.g., evanescent sensors, phase modulation
sensors, and interferometric sensors), optical cavities,
piezo-electric devices, ferroelectric devices, thin film batteries,
and photovoltaic devices. Combinations can also be useful, for
example, the combination of field effect transistors and organic
electroluminescent lamps as an active matrix array for an optical
display.
[0112] A typical photovoltaic cell includes a substrate, a back
contact layer (e.g., molybdenum), an absorber layer (also referred
to as the first semiconductor layer), a buffer layer (also referred
to as the second semiconductor layer), and a top contact layer. The
absorber layer may comprise a thin film of crystalline
Fe.sub.2XY.sub.4 in accordance with the present invention. The
photovoltaic cell can also include an electrical contact of
electrode pad on the top contact layer, and an anti-reflective (AR)
coating on the front (light-facing) surface of the substrate to
enhance the transmission of light into the semiconductor layer.
[0113] One aspect of the present invention is a process comprising
depositing one or more layer(s) in layered sequence onto the
annealed crystalline Fe.sub.2XY.sub.4-containing coating of the
substrate. The layer(s) can be selected from the group consisting
of: conductors, semiconductors, and dielectrics. In one embodiment,
the process provides a photovoltaic device and comprises depositing
the following layers in layered sequence onto the annealed coating
of the substrate having an electrically conductive layer present:
(i) a buffer layer; (ii) a transparent top contact layer, and (iii)
optionally, an antireflective layer.
[0114] Another aspect of the present invention is a device made by
the process comprising depositing one or more layer(s) in layered
sequence onto the annealed crystalline Fe.sub.2XY.sub.4-containing
coating of the substrate.
[0115] Suitable substrate materials for the photovoltaic cell
substrate are as described above and below. The photovoltaic cell
substrate can also comprise an interfacial layer to promote
adhesion between the substrate material and metal layer. Suitable
interfacial layers can comprise metals (e.g., V, W, Cr), glass, or
compounds of nitrides, oxides, and/or carbides.
[0116] Typical photovoltaic cell substrates are glass or plastic,
coated on one side with a conductive material, e.g., a metal. In
one embodiment, the substrate is molybdenum-coated glass.
[0117] Depositing and annealing the crystalline
Fe.sub.2XY.sub.4-containing layer on the photovoltaic cell
substrate to form an absorber layer can be carried out as described
above. A suitable crystalline Fe.sub.2XY.sub.4-containing layer for
a photovoltaic cell comprises a single-formula crystalline
Fe.sub.2XY.sub.4 fraction that is selected from the group
consisting of: crystalline Fe.sub.2SiS.sub.4, crystalline
Fe.sub.2SiSe.sub.4, crystalline Fe.sub.2GeS.sub.4, and crystalline
Fe.sub.2GeSe.sub.4.
[0118] The buffer layer typically comprises an inorganic material
such as CdS, ZnS, zinc hydroxide, Zn (S, O, OH), cadmium zinc
sulfides, In(OH).sub.3, In.sub.2S.sub.3, ZnSe, zinc indium
selenides, indium selenides, zinc magnesium oxides, or n-type
organic materials, or combinations thereof. Layers of these
materials can be deposited by chemical bath deposition, atomic
layer deposition, coevaporation, sputtering or chemical surface
deposition to a thickness of about 2 nm to about 1000 nm, or from
about 5 nm to about 500 nm, or from about 10 nm to about 300 nm, or
40 nm to 100 nm, or 50 nm to 80 nm.
[0119] The top contact layer is typically a transparent conducting
oxide, e.g., zinc oxide, aluminum-doped zinc oxide, indium tin
oxide, or cadmium stannate. Suitable deposition techniques include
sputtering, evaporation, chemical bath deposition, electroplating,
chemical vapor deposition, physical vapor deposition, and atomic
layer deposition. Alternatively, the top contact layer can comprise
a transparent conductive polymeric layer, e.g.,
poly-3,4-ethylenedioxythiophene (PEDOT) doped with
poly(styrenesulfonate) (PSS), which can be deposited by standard
methods, including spin coating, dip-coating or spray coating. In
some embodiments, the PEDOT is treated to remove acidic components
to reduce the potential of acid-induced degradation of the
photovoltaic cell components.
[0120] In one embodiment, the photovoltaic cell substrate coated
with a crystalline Fe.sub.2XY.sub.4-containing film is placed in a
cadmium sulfide bath to deposit a layer of CdS. Alternatively, CdS
can be deposited on the crystalline Fe.sub.2XY.sub.4-containing
film by placing the coated substrate in a cadmium iodide bath
containing thiourea.
[0121] In one embodiment, the photovoltaic cell is fabricated using
a sputtered layer of insulating zinc oxide in place of CdS. In some
embodiments, CdS and ZnO layers are both present in the
photovoltaic cell; in other embodiments, only one of CdS and ZnO is
present.
[0122] In some embodiments, a layer of a sodium compound (e.g.,
NaF, Na.sub.2S, or Na.sub.2Se) is formed above and/or below the
crystalline Fe.sub.2XY.sub.4-containing layer. The layer of the
sodium compound can be applied by sputtering, evaporation, chemical
bath deposition, electroplating, sol-gel based coatings, spray
coating, chemical vapor deposition, physical vapor deposition, or
atomic layer deposition.
[0123] A film fabricated on a substrate as described above can be
incorporated into an electronic device to serve, for example, as an
absorber layer in a photovoltaic device, module, or solar panel.
The typical solar cell includes a transparent substrate (such as
soda-lime glass), a back contact layer (e.g. molybdenum), an
absorber layer (also referred to as the first semiconductor layer),
a buffer layer (e.g. CdS; also referred to as the second
semiconductor layer), and a top electrical contact. The solar cell
may also include an electrical contact or electrode pad on the top
contact layer, and an antireflective (AR) coating on the front
surface of the substrate to enhance the initial transmission of
light into the semiconductor material. FIG. 1 illustrates the above
features in the stack shown therein, which contains the following
elements: transparent substrate 1; back contact layer 2; absorber
layer 3 (which is formed from the Fe.sub.2XY.sub.4 precursors
hereof, including the Fe.sub.2XY.sub.4 nanoparticles described
herein); buffer layer 4; top contact layer 5 [which can be, for
example, a transparent conducting oxide ("TCO") such as zinc oxide
doped with aluminum]; and the electrical contact or electrode pad
on the top contact layer 6.
[0124] The substrate may be made, for example, of a metal foil,
such as titanium, aluminum, stainless steel, molybdenum, or a
plastic or polymer, such as a polyimide (PI), polyamide,
polyetheretherketone (PEEK), polyethersulfone (PES), polyetherimide
(PEI), polyethylene naphthalate (PEN), polyester (PET), or a
metallized plastic. The base electrode may be made of an
electrically conductive material such as a layer of Al foil, e.g.,
about 10 microns to about 100 microns thick. An optional
interfacial layer may facilitate bonding of the electrode to the
substrate. The interfacial (adhesion) layer can be comprised of a
variety of materials, including without limitation chromium,
vanadium, tungsten, and glass, or compounds such as nitrides,
oxides, and/or carbides. The Fe.sub.2XY.sub.4 absorber layer may be
about 0.5 micron to about 5 microns thick after annealing, and more
preferably from about 0.5 microns to about 2 microns thick after
annealing.
[0125] The n-type semiconductor thin film (sometimes referred to as
a junction partner layer) may include, for example, inorganic
materials such as cadmium sulfide (CdS), zinc sulfide (ZnS), zinc
hydroxide, zinc selenide (ZnSe), n-type organic materials, or some
combination of two or more of these or similar materials, or
organic materials such as n-type polymers and/or small molecules.
Layers of these materials may be deposited, for example, by
chemical bath deposition (CBD) and/or chemical surface deposition
(and/or related methods), to a thickness ranging from about 2 nm to
about 1000 nm, more preferably from about 5 nm to about 500 nm, and
most preferably from about 10 nm to about 300 nm. This may also
configured for use in a continuous roll-to-roll and/or segmented
roll-to-roll and/or a batch mode system.
[0126] The transparent electrode may include a transparent
conductive oxide layer such as zinc oxide (ZnO), aluminum doped
zinc oxide (ZnO:Al), indium tin oxide (ITO), or cadmium stannate,
any of which can be deposited using any of a variety of means
including but not limited to sputtering, evaporation, CBD,
electroplating, CVD, PVD, ALD, and the like. Alternatively, the
transparent electrode may include a transparent conductive
polymeric layer, e.g. a transparent layer of doped PEDOT
(poly-3,4-ethylenedioxythiophene), which can be deposited using
spin, dip, or spray coating, and the like. PSS:PEDOT is a doped
conducting polymer based on a heterocyclic thiophene ring bridged
by a diether. A water dispersion of PEDOT doped with
poly(styrenesulfonate) (PSS) is available from H. C. Starck of
Newton, Mass. under the trade name of Baytron.RTM. P. The
transparent electrode may further include a layer of metal (e.g.,
Ni, Al or Ag) fingers to reduce the overall sheet resistance.
Alternatively, the transparent conductor layer may comprise a
carbon nanotube-based transparent conductor.
[0127] The operation and effects of certain embodiments of the
inventions hereof may be more fully appreciated from a series of
examples as described below. The embodiments on which these
examples are based are representative only, and the selection of
those embodiments to illustrate the invention does not indicate
that materials, components, reactants, configurations, designs,
conditions, specifications, steps, techniques not described in the
examples are not suitable for use herein, or that subject matter
not described in the examples is excluded from the scope of the
appended claims and equivalents thereof.
[0128] Although the invention is illustrated and described herein
with reference to specific embodiments, the invention is not
intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims and without departing from the
invention.
EXAMPLES
Example 1
Preparation of Nanoprecursor Mixture to Fe.sub.2SiS.sub.4
[0129] All reagents, except for FeCl.sub.2 were purchased from
Sigma Aldrich and used without further purification. Iron (II)
chloride (FeCl.sub.2, 99.5%) was purchased from Alfa Aesar. The
nanoprecursors (NPs) mixture for the preparation of
Fe.sub.2SiS.sub.4 was obtained by combining 0.253 g of FeCl.sub.2
and 0.128 g of S, each dissolved by sonication in 10 mL of
oleylamine at 104.degree. C., with 0.028 g Si nanopowder with
particle size <100 nm (predispersed in oleylamine at 104.degree.
C.). The dispersions were then combined and reacted at 220.degree.
C.
[0130] The mixtures were further heated for 2 hours under inert
atmosphere (argon). The particles were recovered by precipitation
with anti-solvents followed by centrifugation. Several washing
steps to remove residual organic groups were performed, by
repeating the dispersion precipitation step. The preparation led to
a uniform mixture of nanoparticles of Si and FeS.sub.2, based on a
TEM image of the powder. The particle size of all particles present
in the mixture was below 200 nm.
Example 2
Preparation of Fe.sub.2GeS.sub.4 Ternary Nanomaterial
[0131] Oleylamine solutions, prepared from dissolution of 0.253 g
of FeCl.sub.2, 0.580 g of GeI.sub.4 and 0.128 g of S, each in 10 mL
oleylamine at 104.degree. C., were mixed and heated to 220.degree.
C. for two hours. The particles were recovered by precipitation
with hexanes:ethanol in a 1:1 ratio, followed by centrifugation.
Several washing steps to remove residual organic groups were
performed, by repeating twice the dispersion in toluene and
precipitation in hexanes:ethanol in a 1:1 ratio steps. The
preparation led to a uniform mixture of nanoparticles of Ge and
FeS.sub.2 based on a TEM image of the powder. The particle size of
all particles present in the mixture was below 100 nm. Plate-like
particles have been observed by TEM imaging and the composition was
identified by XPS.
Example 3
Preparation of Amorphous Precursors Mixture to Fe.sub.2GeS.sub.4
Ternary Crystalline Materials
[0132] Oleylamine solutions, prepared from dissolution of 0.253 g
of FeCl.sub.2, 0.580 g of GeI.sub.4 and 0.128 g of S, each in 10 mL
oleylamine at 104.degree. C., were mixed and heated to 220.degree.
C. for one hour. The resulting dark-colored powder was recovered by
precipitation with hexanes:ethanol in a 1:1 ratio, followed by
centrifugation. Several washing steps to remove residual organic
groups were performed, by repeating twice the dispersion in toluene
and precipitation in hexanes:ethanol in a 1:1 ratio steps. The
preparation led to an amorphous mixture based on a TEM image of the
powder. Temperature treatment of this amorphous precursor powder
with stoichiometric composition of Fe:Ge:S of 2:1:4 led to
crystalline Fe.sub.2GeS.sub.4 (as confirmed by XRD).
Example 4
Synthesis of Fe.sub.2GeS.sub.4 Nanoplates by Microwave-Assisted
Synthesis
[0133] Stoichiometric amounts of 353 mg (1 mmol) of Fe(acac).sub.3,
290 mg (0.5 mmol) of GeI.sub.4 and 64 mg (2 mmol) sulfur were
dissolved in 5 mL oleylamine each at 100.degree. C. and added to a
20 mL microwave vial (Biotage) equipped with stirring bar and
containing 5 mL oleylamine heated at 100.degree. C. The vial was
sealed and inserted in a microwave reactor cavity (Biotage
Initiator Reactor). The reaction was carried out at 260.degree. C.
for 30 minutes. The TEM analysis showed fine 2D platelets.
Example 5
Ability of Nanoparticles to Form Crystalline Fe.sub.2SiS.sub.4 and
Fe.sub.2GeS.sub.4
[0134] The precursors' capability of generating crystalline
Fe.sub.2SiS.sub.4 and Fe.sub.2GeS.sub.4 has been characterized by
Temperature Dependent X-Ray Powder Diffraction (TD-XRD). Formation
of crystalline Fe.sub.2SiS.sub.4 and Fe.sub.2GeS.sub.4 was observed
at temperatures as low as 500.degree. C.
Example 6
Deposition of Nanoprecursors Layer
[0135] The nanoprecursors, prepared as dispersions in organic
solvents with sulfur content (such as thiols, dithiocarbamates, and
disulfides), were deposited by spin-coating, spray-coating and
rod-coating, on molybdenum-coated 1''.times.1'' substrates. One to
five layers of nanoprecursor mixtures were deposited for each
sample. The thickness of each coated layer was in the range of
300-500 nm (as determined by SEM analysis of FIB processed
sections).
Example 7
Processing
[0136] The nanoprecursors films (coatings) were subjected to a
thermal treatment step. The annealing procedure was performed in a
tube furnace, under an argon atmosphere and in the presence of
sulfur to preclude sulfur loss. The annealing temperature typically
was in the range of 450.degree. C. to 650.degree. C. The annealed
films were characterized by XPS. Despite air sensitivity indicated
by the presence of oxygen, the films' compositions were nearly
stoichiometric (iron-poor Fe.sub.1.5GeS.sub.4 crystalline
film).
Example 8
Microwave Assisted Processing of Nanoparticle Films
[0137] The nanoparticle films (coatings) prepared as described in
Example 6 were subjected to a thermal treatment step in a modified
microwave furnace. The annealing procedure was performed in a
graphite box, with and without presence of sulfur. The power was
adjusted to correspond to an annealing temperature of 400.degree.
C. to 650.degree. C. The annealed films were characterized by XRD.
Despite air sensitivity indicated by the presence of oxygen, the
films' compositions and purity were confirmed.
Example 9
Microwave Assisted Processing of Amorphous Powders Films
[0138] The amorphous powders films (coatings) prepared as described
in Example 6 were subjected to a thermal treatment step in a
modified microwave furnace. The annealing procedure was performed
in a graphite box, with and without presence of sulfur. The power
was adjusted to correspond to an annealing temperature of
400.degree. C. to 650.degree. C. The annealed films were
characterized by XRD. Despite air sensitivity indicated by the
presence of oxygen, the films' compositions and purity were
confirmed.
Example 10
Fabrication of Solar Devices
[0139] Device fabrication followed the structure of CIGS using CdS
as buffer layer or ZnS. The CdS layer deposition was performed by
chemical bath deposition and ZnS was deposited by vacuum
sputtering. Diode behavior and small photocurrents (0.003 mA) were
recorded.
* * * * *